Remove, as a knot (4)
I believe the answer is:
undo
'remove as a knot' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for undo that I've seen before include "Release fastening" , "Release button, knot etc" , "Loosen; annul" , "Ruin or undermine" , "Destroy - nullify" .)